1. Field of the Invention
The subject invention relates to a method of removing halides from carboxylic acids to produce essentially halide-free carboxylic acid. The invention also relates to a method of preparing both gelular and macroreticular silver(I)-exchanged resins.
2. Description of Related Art
Commercial production of unsaturated esters is carried out by catalytically reacting the corresponding carboxylic acid with an alkene and oxygen in the presence of a noble metal catalyst. For example, vinyl acetate is produced by reaction of acetic acid with ethylene in the presence of the noble metal catalyst and oxygen. To avoid premature deactivation and maximize the life of the noble metal esterification catalyst, it is desirable that the reactants be essentially free of halides, particularly iodides.
While the alkene reactant is essentially halide-free, halides may be introduced into the esterification reactor with the carboxylic acid. For example, in one commercially accepted procedure for making carboxylic acids, an alcohol such as methanol is carbonylated in the presence of a halide-promoted rhodium catalyst to produce a carboxylic acid such as acetic acid. Typically, the halide promoter is a bromide or an iodide. It is not unusual, therefore, for a small amount of the halide promoter or a derivative thereof to be recovered with the acetic acid prepared in the fashion. Unless removed by treatment, the halide accompanies the acid into the oxidative esterification reactor.
Unfortunately, the concentration of halide in the carboxylic acid can be very low and still unacceptably shorten the life of the esterification catalyst. For example, one part per billion (ppb) of iodide has a deleterious effect, substantially shortening the useful life of the noble metal esterification catalyst used in preparing vinyl acetate. Therefore, iodide removal must be essentially quantitative to be effective in prolonging catalyst life.
Pohl and Johnson ("Ion Chromatography--The State-Of-The-Art," 18 J. Chromat. Sci. 442 (1980)) describe using silver(I)-exchanged resin as a suppressor in anion ion chromotography to modify the chromatographic eluent and achieve a sensitive detection of weakly electrolytic ionic species. The resin is used to remove high concentrations of interfering halides (e.g., brines) from the eluent. Microporous rather than macroporous exchange resins are preferred as suppressors because macroporous resins strongly absorb weak electrolytes.
Hingorani and Venkateswarlu ("Removal Of Radioactive Iodine and Methyl Iodide by use of Silver-impregnated Resin," 12 Chem. Eng. World 59 (1977)) disclose a method of batchwise removal of radioactive iodine and labelled methyl iodide from aqueous solutions using a silver-impregnated gelular resin. The aqueous solutions were intended merely as an expedient during experimentation directed toward removal of iodine and methyl iodide from waste streams obtained from plants which reprocess irradiated fuel elements. This method is unsatisfactory for removing iodide from carboxylic acids used to produce commercial quantities of vinyl carboxylates. First, the method proceeds at a slow rate. Even after more than 110 hours, only about 83% of the iodide in the solution was absorbed--96% removal requires about 150 hours of treatment. Hingorani et al suggest that the absorption rate is low in their process because methyl iodide is not easily hydrolyzed in aqueous solution. Also, continuous processes are preferred to batch techniques for commercial use because batch techniques require extremely large vessels, catalyst charges, holding tanks, and the like. Thus, one skilled in the art would not consider this technique commercially practicable for treating carboxylic acids used to produce large quantities of unsaturated esters.
Preparation of a silver-substituted cation exchange resin as used by Hingorani also has heretofore been difficult and inefficient. A cation exchange resin typically is soaked overnight in a solution of silver nitrate (AgNO.sub.3). The exchange of silver onto the resin produces nitric acid as a byproduct. Nitric acid is both a strong acid and a strong oxidant. As the exchange reaction progresses, the increasing activity due to the accumulating nitric acid causes the equilibrium exchange reaction to stop when about 5% of the silver still remains in solution. Further, the oxidizing power of the byproduct nitric acid requires that the exchange resin be washed thoroughly before use, to remove substantially all of the nitric acid.
Applicant has determined that cationic gelular resins, such as Amberlite.RTM. IR-120 used by Hingorani et al, are not suitable for removing halides from carboxylic acids because carboxylic acids cause such resins to shrink, making internal active sites inaccessible to reactants and increasing pressure drop in a fixed-bed system. To achieve reasonable reaction rates, relatively high temperatures, i.e., at least about 60.degree. C., must be utilized. However, high temperature promote corrosion of equipment, releasing polyvalent metal ions into the carboxylic acid stream. Polyvalent metal ions are significant contributors to resin degradation.
It is an object of this invention to provide a method for essentially complete removal of halide impurities from carboxylic acid by contacting halide-containing acid with a silver-exchanged macroreticular resin.
It is a further object of this invention to provide an improved method for preparing macroreticular silver-exchanged resins suitable for use in the present invention.